It has long been a practice to form underwater transmitter or projector elements of active piezoelectric materials such as barium titanate in an annular or cylindrical configuration and polarized such that, when energized, they expand and contract radially with the applied signals. It has also been known to form the elements in segments which are cemented together and in which electrodes are formed in the joints. Such an annular element will resonate at that frequency at which the mean circumference is equal to one wave length in the ceramic material. Thus the mean diameter will vary directly as the velocity of sound for a constant resonant frequency. For an annular element of a given material, then, the desired resonant frequency controls the diameter of the projector.
Because of its desirable qualities, barium titanate is often preferred as a projector material. It has the highest velocity of sound of conventional piezoelectric ceramic materials, and for a 10 KHz element the maximum diameter possible for an annular projector as described above is approximately 5.8 inches. The individual annular elements are normally axially arranged in a stack, and the small central aperture severely limits the space available inside the elements for structure and electrical circuitry. Consequently the projector elements have usually been displaced axially from the receiving hydrophones which are normally fabricated in arrays of considerably greater diameter to improve directional sensitivity. This tends to cause the entire projecting and receiving transducer assembly to be longer than is desirable. For many applications, it would be desirable for both operational and structural reasons to be able to fabricate the projectors of substantially greater diameter than would normally result from the materials used and the frequencies desired.